1. Field of the Invention
The present invention relates to a power supply system and a vehicle provided with the system and, more specifically, to a power supply system including a power storage unit and a vehicle provided with the same.
2. Description of the Background Art
Recently, considering environmental issues, vehicles using electric motors as driving power sources such as electric vehicles, hybrid vehicles and fuel cell vehicles have attracting attention. On such a vehicle, a power storage unit formed by a secondary battery or an electric double layer capacitor is mounted, for supplying electric power to the motor or for converting kinetic energy to electric energy for storage at the time of regenerative braking.
In order to improve running performance of such a vehicle, the power storage unit must be of high reliability. Meanwhile, charging/discharging performance of the power storage unit much depends on the state of use, and over-charging or over-discharging leads to degradation.
Therefore, charge/discharge control of the power storage unit for reducing degradation of power storage unit has been proposed. By way of example, Japanese Patent Laying-Open No. 10-164761 discloses a control structure for a battery mounted on a hybrid vehicle, in which conditions for starting and ending battery charge are found not to cause internal heat generation at the power storage unit.
According to the disclosure, in order to reduce battery degradation caused by increased temperature, conditions for starting charging with optimal charge/discharge period in accordance with ambient temperature in a vehicle are found to be the battery charge starting conditions. Further, in order to reduce battery degradation caused by increased temperature, conditions for ending charging with optimal charge/discharge state (depth of discharge) in accordance with ambient temperature in a vehicle are found to be the battery charge ending conditions.
For a hybrid vehicle in which internal combustion engine and electric motor are efficiently combined for running, a structure has been proposed in which the power storage unit mounted on the vehicle is electrically connected to an external power source such as commercial power supply through a connector, to enable charging of the power storage unit from the external power source. When the power storage unit is charged beforehand from the external power source, running for a short distance such as commuting or shopping becomes possible with the internal combustion engine stopped, and general fuel consumption efficiency can be improved. Such charging of the power storage unit from external power source is sometimes referred to as external charging.
On the other hand, if a hybrid vehicle with such external charging mode is externally charged while the vehicle system is stopped, it follows that the power storage unit is left unused with high SOC (State of Charge) value, from the end of external charging to the next activation of the vehicle system.
Degree of degradation of power storage unit such as a secondary battery much differs dependent on the environment of usage, and degree of degradation may increase at a high temperature or at a high SOC state. Therefore, if the power storage unit is left unused in the high SOC state after completion of external charging, degradation of power storage unit is disadvantageously accelerated. Japanese Patent Laying-Open No. 10-164761 mentioned above is silent about any solution to such a problem.